1. Field of the Invention
The present invention relates to injection molding machines. More particularly, the present invention relates to a two-stage, electrically-operated injection molding machine wherein plastication is performed in a first processing stage and injection is performed in a second processing stage that is separate from the first stage.
2. Description of the Related Art
The injection unit of an injection molding machine provides essentially two functions during the course of a normal operating cycle: namely, plastication of the molding material, followed by injection of the material into a mold cavity. In a reciprocating screw injection molding machine, sometimes referred to herein as a "single-stage machine," both the plastication and the injection of the molding material are performed by the same apparatus. The plastication function is accomplished by introducing plastic material in pelletized form, and rotating the plastication screw within a heated barrel. The plastic material is heated and worked until it becomes molten and is transported by the screw in a generally helical direction toward the forward end of the screw. The molten material moves toward the forward end of the screw to exert a rearward pressure or force on the screw that serves to move the screw axially within the barrel, away from the forward end of the barrel. When the desired quantity of molten plastic material has been accumulated at the forward end of the screw, an amount commonly referred to as a "shot", the retracted plastication screw is moved rapidly forward, without rotation, to inject the molten molding material into a mold cavity defined by a closed mold to form a desired molded part.
A molding machine can also be provided wherein the plastication and injection functions are performed by separate elements of the machine. In such a machine, sometimes referred to herein as a "two-stage machine," the plastication function is still performed by a plastication screw that is rotated within a heated barrel, but the screw is not axially shifted to define the shot and to inject the shot into the mold cavity. Instead, the molten plastic material is caused to flow into a separate and distinct accumulator that is usually positioned adjacent the plastication barrel. When the desired amount of molten molding material has been received within the cylindrical barrel of the accumulator, a ram or plunger carried within the barrel is moved axially to inject the molten plastic melt from the accumulator into the mold cavity.
The advantages of a two-stage injection unit include more uniform plastication of the molding material, reduced wear of the plastication screw and plastication barrel, and the potential for higher injection pressures than are attainable in a single stage machine. The primary disadvantages of the two-stage injection unit are higher equipment cost and the tendency for some uninjected material to remain within the accumulator barrel as carryover material. Because the carryover material can be subjected to high temperatures for a longer time period, thermal degradation of the carryover material can occur, which can adversely affect the quality of the molded part.
In recent years, electric motors have been employed in place of hydraulic motors, as the direct power source for some reciprocating screw injection units. Electrically-operated systems have sometimes utilized separate motors for each function of the molding material injection unit: one motor solely for rotating the feed screw for plastication, and a second, separate motor coupled with a mechanical mechanism, such as a ball screw, roller screw, rack and pinion or similar device, to convert the rotary motion of the second motor into the linear motion required to move the screw axially for injection. Electric motors driving screw mechanisms can also be used as power sources in machines having two-stage injection units.
Drive systems based upon electric motors and ball screws, for example, have their practical limits. For example, the larger diameter ball screws required to provide the necessary high injection pressures for larger sized parts are quite expensive. In addition, since the availability and cost of ball screws are affected more by ball screw diameter rather than by length, ball screws in excess of six inches in diameter are virtually unavailable in commercial quantities, which has severely limited the maximum capacity of an all-electric injection unit to about a 40 ounce shot size. Although a large diameter ball screw can be replaced by two smaller diameter ball screws operating in tandem, the cost for plural ball screws and associated components is also quite high. In fact, the construction of electric reciprocating screw injection units that have capacities to match the range of shot sizes of commercially available hydraulically-powered units would also be very high cost for other, similar drive mechanisms, such as roller screws or rack and pinion. Furthermore, large capacity units would require ball screws of diameters that are untested and that, in fact, exceed current ball screw manufacturing capabilities. Similar problems are incurred with alternative rotary to linear motion converters that could be used to drive a reciprocating screw.
Accordingly, it is an object of the present invention to provide an improved drive apparatus for an injection molding machine having a two-stage injection unit.
It is another object of the present invention to provide a two-stage injection unit that is electrically-powered, that is simple in construction, and that has no practical limits (upper or lower) on shot capacity.